1. The Field of the Invention
This invention relates to fabrication of semiconductor devices and, more particularly, to a method of forming a photoresist pattern using a resist reflow technique.
2. Description of the Related Art
Due to ever-increasing density in ULSI (ultra-large-scale integration) circuits, various techniques have been proposed to overcome the limits of the present semiconductor fabrication technology. For example, reduction in size of a contact hole has been actively pursued below the resolution limits of the current optical lithography technology.
To form a contact hole, optical lithography technology has been conventionally used to form a photoresist pattern 100. Using the photolithography technique, patterns of a mask are transferred onto a photoresist applied on a semiconductor substrate 102 including an insulating layer. The photoresist is subjected to development after patterns are transferred thereto. Thereafter, the underlying insulating layer is etched using the developed photoresist pattern.
In recent years, however, the minimum feature size of the advanced ULSIs has reached the resolution limits of the conventional optical lithography technology. For example, it is known that i-line lithography technology is adequate for forming a contact hole having a minimum feature size of over 0.30-0.35 xcexcm. Thus, with current i-line or Krf optical lithography systems, the minimum feature size below sub-0.18 xcexcm would be very difficult to achieve.
There have been attempts to solve the resolution problem. Particularly, the extension of the optical lithography technology for sub-0.18 xcexcm contact hole lithography has been attempted. These attempts include: 1) Phase shift mask (PSM) technology, 2) Off-Axis illumination (OAI) technology, 3) resist reflow technology and 4) a multi-layer resist process.
Among these technologies, the resist reflow technology is potentially an excellent candidate because of its simplicity. One such resist reflow technology is disclosed in U.S. Pat. No. 4,022,932. In this technology, upon exposure to a vapor of a solvent, the photoresist flows apertures in the photoresist layer as a result of the increase in the volume of the photoresist and the decrease of the viscosity of the photoresist. As a result, the area of the apertures can be reduced. However, critical dimension (CD) control is very difficult in this technology because severe overhang often results in the photoresist contact hole pattern, as illustrated in FIG. 1. Worst case, the photoresist contact hole pattern could collapse at high temperatures. Another resist reflow technology is described in an article by Takahiro Yamauchi et al, entitled xe2x80x9c0.2 xcexcm Hole Pattern Generation by Critical Dimension Biasing Using Resin Overcoat,xe2x80x9d Jpn. J. Appl. Phys. Vol. 35 (1995) pp. 6615-6626, Part 1. No. 12B, December 1995. That method, illustrated here in FIG. 2, uses i-line resist and polymethyl methacrylate (PMMA). First, a novolak-based photoresist hole pattern 14xe2x80x2 is formed on a semiconductor substrate 16, using resist coat/bake, and image expose/develop as illustrated. Then, PMMA 18 is coated over the photoresist hole pattern 14xe2x80x2 by spin coating. Next, the resulting structure is baked to form an interfacial layer 20 between PMMA 18 and the hole pattern 14xe2x80x2. The interfacial layer is formed of a mixture of PMMA and the novolak resin. This interfacial layer defines the critical features. Subsequently, as shown in FIG. 2, using deep-UV flood exposure and xylene rinse, pure PMMA is stripped. The interfacial layer 20 remains on the side walls of the photoresist hole pattern 14xe2x80x2, thereby shrinking the diameter of the photoresist hole pattern 14xe2x80x2. The interfacial layer 20 consequently functions as an etching mask to form a contact hole during a contact hole etching process.
Unfortunately, in this method, the cleaning steps for removing PMMA by deep-UV flood exposure and xylene rinse have made the overall process very complicated. In addition, by-product particles indicated by B undesirably left on the interfacial layer after the cleaning steps, and the difficulty of controlling the formation of the interfacial layer itself render the control of critical dimension (CD) very difficult. Further, because xylene is highly toxic, the use of xylene can cause various environmental problems. For these reasons, the method described in FIG. 2 has not been practically implemented.
More recently, Mitsubishi and Clariant developed contact hole shrinking methods, called RELACS (Resolution Enhancement Lithography Assisted by Chemical Shrink), which is described in an article by Takashi Kanda et al, entitled xe2x80x9c100 nm Contact Holes Using Resolution Enhancement Lithography Assisted by Chemical Shrink,xe2x80x9d Microlithography World, Autumn 1999. This method uses organic chemical material that thermally cross-links with protons remaining on the walls of the patterned photoresist. This cross-linked area defines the critical features. However, many additional baking steps and photoresist development steps are also required in this method. Further, because the degree of cross-linking is not easily controlled, there have been problems in CD control. Also, undesirable reactants are left on the side walls of the photoresist pattern, which adversely affects controlling CD. For these reasons, practical implementation of the above-described resist flow technologies has not yet been very successful.
Accordingly, a need exists for a simplified and more practical method of forming a photoresist pattern for highly integrated ULSI circuits under the 0.18 xcexcm design rule (and below) using the resist reflow technique. Particularly, a need exists for a method allowing precise CD control using conventional optical lithography equipment such as an inexpensive i-line lithography system.
The present invention relates to a method of forming a photoresist contact hole pattern using water-soluble organic over-coating material (WASOOM). According to the preferred embodiments of the present invention, a photoresist pattern that defines an opening therethrough is provided over a semiconductor substrate surface, which can include an insulating layer. Then, a layer of water-soluble organic over-coating material (WASOOM) is coated over the photoresist pattern including the opening thereof. Next, the resulting structure is flowed to shrink the size of the opening. After the resist reflow, WASOOM is removed simply by a hydrophilic-solution such as DI water. Eventually, the insulating layer is etched to form a contact hole therein, using the shrunken-opening photoresist pattern as an etching mask.
Thus, using the methods of the present invention, a photoresist pattern capable of forming a minimum feature size of 0.18 xcexcm and below can be formed using a conventional optical lithography system.
Further, because WASOOM is water-soluble, WASOOM can be substantially completely removed from the photoresist pattern using a simple cleaning process, i.e., water rinse, after baking for resist reflow. During the reflowing step, the WASOOM plug limits reflow of resist into the opening. Thus, the process steps are simplified and the problems such as the difficulty in CD control and the environmental issues are avoided.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention that proceeds with reference to the accompanying drawings.